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PubMed1994

Effects of attention on perception of features and figural organisation.

Four studies are reported that demonstrate effects of attention on early perceptual processes. These studies show that directing attention to stimulus location influences perceived brightness, perceived length, and the overall organisation of the Müller-Lyer figure and ambiguous figures. Since in all four studies the dependent measure was dissociated from the attentional instructions, the results are not open to traditional criticism of late-selection advocates, and therefore provide strong support for early-selection views of attention.

Attention↗

PubMed1994

Lightness perception can be affected by surface curvature from stereopsis.

It is demonstrated that lightness perception can be affected by shape from stereopsis. The starting point was a report by Knill and Kersten that the perceived lightness of a monocularly viewed surface can be affected by outline-contour cues indicating that the surface is three-dimensional (3-D). In that study stimuli consisted of two equally sized abutting regions each having the same vertical linear-intensity ramp, so that the horizontal abutting boundary of the two patches created a sharp change in intensity. When this version of the Craik-O'Brien-Cornsweet stimulus has a rectangular outline, it exhibits the standard simultaneous contrast illusion: equivalent patches in the top and bottom regions appear to have different brightness despite having the same luminance. Knill and Kersten replicated this phenomenon with stimuli whose outline-contour cues were consistent with a flat (planar) surface. They found, however, that the illusion was greatly reduced in stimuli with outlines consistent with two abutting 3-D quarter cylinders, for which equivalent regions in the two halves appeared of similar lightness. Knill and Kersten interpreted this effect in terms of surface-lightness computations that took into account 3-D surface shape to achieve an integrated interpretation of the luminance and shape data. In the present report three experiments are described for which these earlier findings were taken as the starting point. In the first experiment the results were replicated by the use of a different methodology. In the second experiment it was shown that shape-from-stereo can produce similar effects on lightness perception to that caused by shape-from-contour. Real 3-D objects with curved surfaces, luminance profiles of the Knill and Kersten type, and carefully controlled outline-contour cues were used so that the objects appeared flat when viewed monocularly but curved in 3-D when seen binocularly. The third experiment was a control confirming that the stereo effect was not simply due to differences caused by monocular versus binocular viewing. It is concluded that the human visual system uses stereo cues, as well as outline-contour cues, in the interpretation of luminance data to recover surface lightness.

Adult↗

PubMed1994

Ambiguous pictorial depth cues and perceptions of nonrigid motion in the three-loop figure.

The three-loop figure is a two-dimensional (2-D) pattern that generates (mis)perceptions of nonrigid three-dimensional (3-D) structure when rotated about its centre. Such observations have been described as counterexamples to the principle whereby a moving object is presumed to be rigid, provided that a rigid interpretation is possible (ie the 'rigidity constraint'). In the present investigation we demonstrated that stationary three-loop figures exhibit many of the classic properties of multistable/ambiguous figures, with any one of several possible 3-D configurations being reported at any one instant. Further investigation revealed that perceived nonrigidity during rotation was markedly reduced (and rigidity enhanced) when the figure was modified with static pictorial depth cues (eg shading, interposition). These cues had no effect on the overall proportion of time that observers reported 3-D organisations in stationary versions of the figure, but significantly reduced the frequency of perceptual reorganisation, and increased the duration for reporting a particular organisation. Since each of the perceived 3-D structures in a stationary ambiguous 2-D figure has a unique kinetic counterpart (ie rigid transformation), we attribute the nonrigid structure perceived when the figure rotates to the integration of these otherwise inconsistent kinetic components; and have further illustrated this with modified versions of a Penrose impossible triangle. Under kinetic versions of the classical size/distance invariance hypothesis, the rigidity constraint may be considered to represent a special instance of size/shape constancy, in which case counterexamples involving (mis)perceptions of nonrigid structure are comparable to other well-known exceptions to such principles of minimum object change (eg classical illusions).

Cues↗

PubMed1994

Temporal properties of colour and shape priming: evidence of multiple components of attention.

It has been shown in several studies that the selection of letters or numerals from an array can occur efficiently if criteria such as location, colour, shape, or size are used. It is also known that there are at least two components of spatial attention, the transient exogenous and the sustained endogenous. An interesting question is whether it is possible to elicit similar components when colour or shape information is used as a selection criterion. Experiments are reported the aim of which was to try to produce colour or shape precue effects on location and colour/shape discrimination accuracy, and to determine the minimum latency of these effects by sampling numerous cue lead times from 0 ms to 1000 ms. The results showed that, although accuracy did not increase monotonically with increased cue lead time, there were two transient peaks of performance at cue lead times of 0-200 ms, culminating at 30-50 ms and at 160-200 ms. It is suggested that the first component reflects nonspecific exogenous activation, triggered by the onset of a cue; the second component, in turn, may reflect feature-specific endogenous activation related to selection by colour or shape.

Attention↗

PubMed1994

Stereokinetic effects with sharp and fuzzy illusory contours.

A black, 8-shaped pattern, whose centre of gravity is in the centre of a rotating disc, appears to split into two black discs rotating with phenomenal independent motion, orientation stability, and sliding of one on the other. The type of observed movement, the order of overlapping, and the extent of the stereokinetic depth in relation to the contour type and different dimensions of the pattern were investigated. The experimental data show that a fuzzy contour facilitates the stereokinetic effect. Furthermore, the extent of the stereokinetic depth has been found to be greater in case of fuzzy contour and vertical orientation of the pattern. These results are in contrast with the models in which this effect is considered as a problem of minimisation of distances or velocities. A vector model of the observed movement and an interpretation of the overlapping based on an energy approach are proposed. A tentative explanation of the stereokinetic depth for our patterns is put forward.

Adult↗

PubMed1995

Depth relief.

A study is reported of the depth relief in a simple three-dimensional scene consisting of a white, rough sphere on a planar support, illuminated in a natural manner. Viewing conditions included monocular and binocular as well as 'synoptical' viewing. In the synoptical condition the eyes are optically superimposed. The local surface attitude was probed via a gauge figure that had to appear as a circle painted upon the surface of the object. The measurements allow a reconstruction of the depth relief, which can be compared with the (known) range map. Idiosyncratic differences between three subjects were found that are the exact reverse of what was found for these same subjects for the case of pictorial relief reported in an earlier paper. Apparently the subjects put different weights on sources of sometimes conflicting, sometimes corroborative evidence. The relief is deepest for binocular vision, flatter for monocular vision, and flatter still for synoptical vision. Deviations from veridicality that cannot be explained by mere depth scaling exist and may be due to the nature of the monocular cues.

Cues↗

PubMed1995

Integration by association: combining three-dimensional cues to extrinsic surface shape.

Surface shape is visually derived from multiple sources of three-dimensional (3-D) information. The apparent unity of the percept creates an assumption that there are perceptual processes that resolve and combine the different channels of information within some common 3-D representation. However, integration by the explicit conversion of the information from different sources into a common representation is computationally difficult and unnecessary. An alternative, integration by association, is discussed and related to a variety of experimental results on 3-D-cue conflict.

Association↗

PubMed1995

The perceptual flattening of three-dimensional scenes enclosed by a frame.

The effects of a visible frame around a three-dimensional scene on perceived depth within the scene was investigated in three experiments. In experiment 1 subjects judged the slant of an object that had been rotated about a vertical axis. Judged slant was reduced when the frame was illuminated. In experiments 2 and 3 subjects judged the shape (width-to-height ratio) of the object. The object was judged to be narrower when the frame was illuminated (experiment 2) or when a frame was added to the scene in an illuminated room (experiment 3). These results demonstrate that the presence of a frame around a three-dimensional scene serves as a flatness cue, reducing perceived depth within the scene.

Analysis of Variance↗

PubMed1995

Phenomena of illusory form: can we bridge the gap between levels of explanation?

The study of illusory brightness and contour phenomena has become an important tool in modern brain research. Gestalt, cognitive, neural, and computational approaches are reviewed and their explanatory powers are discussed in the light of empirical data. Two well-known phenomena of illusory form are dealt with, the Ehrenstein illusion and the Kanizsa triangle. It is argued that the gap between the different levels of explanation, bottom-up versus top-down, creates scientific barriers which have all too often engendered unnecessary debate about who is right and who is wrong. In this review of the literature we favour an integrative approach to the question of how illusory form is derived from stimulus configuration which provide the visual system with seemingly incomplete information. The processes that can explain the emergence of these phenomena range from local feature detection to global strategies of perceptual organisation. These processes may be similar to those that help us restore partially occluded objects in everyday vision. To understand better the Ehrenstein and Kanizsa illusions, it is proposed that different levels of analysis and explanation are not mutually exclusive, but complementary. Theories of illusory contour and form perception must, therefore, take into account the underlying neurophysiological mechanisms and their possible interactions with cognitive and attentional processes.

Arousal↗

PubMed1995

Foreshortening in cube drawings by children and adults.

The lengths of lines and the sizes of angles were measured in freehand drawings of cubes produced by 190 children and 158 adults. The lengths of oblique lines depicting receding cube edges were foreshortened relative to horizontal lines showing nonreceding edges. In the drawings from children aged 9 and 10 years the obliques were foreshortened by about 40%, compared with 30% in adults' drawings. The amount of foreshortening was not correlated with the angle at which the obliques were drawn. Line lengths were also foreshortened in "transparent" drawings, which are often said to show "what is known" about cube structure. One explanation of the results is that line lengths are foreshortened by children and adults to create the visual impression of equal-length cube edges, even in transparent drawings. This foreshortening fits with how the receding edges of a square face of a cube project to a vantage point. Children and adults who use foreshortening are not depicting the structure of cubes by matching features such as equal-length edges with similar features in the physical lines on the page, as has been suggested in some recent approaches to drawing development.

Adolescent↗

PubMed1997

Occlusion constraints and stereoscopic slant.

In binocular vision horizontal magnification of one retinal image leads to a percept of three-dimensional slant around a vertical axis. It is demonstrated that the perception of slant is diminished when an occlusion interpretation is possible. A frontoparallel plane located in the immediate vicinity of a slanted surface in a location which allows a perception of occlusion reduces the magnitude of perceived slant significantly. When the same plane is placed on the other side, the slant perception is normal because there is no alternative occlusion interpretation. The results indicate that a common border between the occluder and a slanted surface is not a necessary condition for the reduction effect. If the edges are displaced and the edge of the slanted surface is placed in a location in which it could be occluded, the effect still appears.

Depth Perception↗

PubMed1997

Shading and stereo in early perception of shape and reflectance.

Recent experiments involving shaded 2-D stimuli have shown that early-vision mechanisms are capable of interpreting 3-D shape from shading. In particular, target discrimination tasks suggest that a target 'pops out' when background distractors, but not the target, can be interpreted as convex and lit from above or top-left. Since the problem of extracting 3-D shape from shading is intrinsically ill-defined, early vision may need to make these twin assumptions of convexity and top-left lighting in order to constrain the problem. Would these assumptions be recognized as unnecessary and consequently discarded when 3-D shape could be unambiguously defined by some other cue, like stereo disparity? A 2AFC stimulus onset asynchrony paradigm with masking was used in target discrimination experiments. The performance of five naive subjects on tasks where only shading cues were present was compared with that on tasks involving shading as well as stereo cues that define shape unambiguously. The results show that although stereo disparity information is incorporated by early-vision 3-D mechanisms, it is not used to overturn the default assumptions of lighting and shape. Stereo information is interpreted within the framework of top-left lighting, and a consistent preference for convexity is seen over concavity.

Contrast Sensitivity↗

PubMed1997

Effects of texture, illumination, and surface reflectance on stereoscopic shape perception.

Observers viewed computer-generated stereograms of randomly structured smooth surfaces and were required to judge the perceived local orientation at numerous probe points by adjusting a monocular gauge figure. The surfaces were depicted with specular or Lambertian reflectance functions, either with or without identifiable texture elements, and with varying directions of illumination. The results revealed a strong linear correlation between the judged patterns of relief and the actual depicted objects, though there were systematic differences in the magnitude of depth scaling in the different conditions. In general, the accuracy and reliability of observers' judgments for the smoothly shaded shiny surfaces was slightly lower than for the textured surfaces and slightly higher than for the smoothly shaded Lambertian surfaces. The direction of illumination had no detectable effect on the observers' judgments.

Computer Graphics↗

PubMed1997

Spatial scale in stereo and shape from shading: image input, mechanisms, and tasks.

The problem of spatial scale in depth perception can be considered at three stages: image input, mechanisms, and depth descriptors specific for certain tasks. A review is presented of a number of earlier experiments supporting the distinction between a coarse, correlation-based mechanism and a feature-matching mechanism of stereopsis in terms of their respective inputs and outputs. In order to measure the influence of the correlation mechanism on the perception of three-dimensional shape, a shape-probe experiment was designed. For smooth intensity profiles with constant disparity, the results show that perceived shape is largely independent of overall disparity but does make use of shape from shading as well as the assumption that brighter parts are in front (proximity-luminance covariance).

Depth Perception↗

PubMed1997

Moving stimuli define the shape of stationary chromatic patterns.

A study is reported of phenomena involved in perceptually unified organisation of a stationary chromatic pattern and a moving black outline or dot pattern. When the corners of the outline pattern were temporally oscillated on a stationary chromatic square, the chromatic border appeared to follow the moving outline, as if captured by it. This capture effect was also observed with moving dots: the chromatic border was defined by an imaginary line connecting the moving dots. Both capture effects occur over a region that becomes wider with increasing velocity of the oscillation. These observations suggest that the visual system effectively uses information from moving features to define the shape of overlapping chromatic image regions.

Color Perception↗

PubMed2001

Efficient search for size targets on a background texture gradient: is detection guided by discontinuities in the retinal-size gradient of items?

Six visual search experiments were carried out to investigate the processing of size information in early vision. The apparent size of display items was manipulated independently of their retinal size by placing items on a textured surface which altered the perceived distance in depth of the items. Overall, these experiments demonstrate that a target item differing from non-target items in terms of apparent size can be detected efficiently. However, the pattern of results indicates that, rather than deriving apparent-size information, target detection is guided by discontinuities in the 'retinal-size gradient' of items, in particular between items at the same 'depth'. Although the arrangement of items on the texture surface strongly influenced search, this was largely due to the retinal size of items and the retinal separation between items. The implications of these experiments for the nature of the pre-attentive representation of size are discussed.

Depth Perception↗

PubMed1998

Reproduction of object shape is more accurate without the continued availability of visual information.

An unfamiliar configuration lying in depth and viewed from a distance is typically seen as foreshortened. The hypothesis motivating this research was that a change in an observer's viewpoint even when the configuration is no longer visible induces an imaginal updating of the internal representation and thus reduces the degree of foreshortening. In experiment 1, observers attempted to reproduce configurations defined by three small glowing balls on a table 2 m distant under conditions of darkness following 'viewpoint change' instructions. In one condition, observers reproduced the continuously visible configuration using three other glowing balls on a nearer table while imagining standing at the distant table. In the other condition, observers viewed the configuration, it was then removed, and they walked in darkness to the far table and reproduced the configuration. Even though the observers received no additional information about the stimulus configuration in walking to the table, they were more accurate (less foreshortening) than in the other condition. In experiment 2, observers reproduced distant configurations on a nearer table more accurately when doing so from memory than when doing so while viewing the distant stimulus configuration. In experiment 3, observers performed both the real and imagined perspective change after memorizing the remote configuration. The results of the three experiments indicate that the continued visual presence of the target configuration impedes imaginary perspective-change performance and that an actual change in viewpoint does not increase reproduction accuracy substantially over that obtained with an imagined change in viewpoint.

Depth Perception↗

PubMed1998

Types of shear disparity and the perception of surface inclination.

A study is reported of (i) the perceived inclination of a textured surface in depth about a horizontal axis as a function of disparity magnitude for horizontal-shear disparity, vertical-shear disparity, and rotation disparity; and (ii) interactions between patterns with shear or rotation disparity and superimposed or adjacent patterns or lines with zero disparity. Horizontal-shear disparity produced strong inclination which was enhanced by superimposed or adjacent zero-disparity stimuli. It produced little or no inclination contrast in superimposed or adjacent zero-disparity stimuli. Vertical-shear disparity produced inclination in the opposite direction (induced effect) which was reduced to near zero by a superimposed zero-disparity pattern. Adjacent vertical-shear and zero-disparity patterns appeared inclined at slightly different angles with a wide curved boundary. This suggests that vertical-shear disparities are averaged over a wide area. Rotation disparity produced minimal inclination. A superimposed or adjacent zero-disparity line appeared strongly inclined. A superimposed or adjacent zero-disparity pattern appeared vertical and caused the pattern with rotation disparity to appear inclined. Four mechanisms are proposed to account for the results: depth contrast, depth enhancement, deformation-disparity processing, and disparity transfer arising from cyclovergence.

Computers↗